The present invention disclosed herein relates to a semiconductor laser, and more particularly, to a dual mode semiconductor laser and a terahertz wave apparatus using the same.
Many researches with respect to a terahertz (THz) wave having a frequency band of about 0.1 THz to about 10 THz are being conducted in recent years. The terahertz wave has proper characteristics as followings. The terahertz is harmless to humans due to its low energy. Also, many molecules have their proper spectrums in a terahertz wave band. A time-domain spectroscopy (TDS) system using a femtosecond (fs) laser was mainly used as a terahertz wave generation apparatus.
The TDS system excites carriers in a photoconductive switch of a photomixer using the femtosecond laser. The photoconductive switch is formed of a material having a very short carrier lifetime. As a result, a current having femtosecond pluses flows into an antenna. Here, when the pulse-type current flows into the integrated antenna, a THz wave having a broad band is generated and is propagated through the air. Thus, the TDS system basically uses the pulse-type THz wave. However, there is a disadvantage in that the femtosecond (fs) laser has a large size and expensive. In case of various applications used in fields, a system, which has a small size and portable and may obtain results in a frequency band using a THz continuous wave (CW THz), but a time-domain spectroscopy, is useful.
Thus, a photomixing method using a photoconductive antenna or a photomixer and a laser having two wavelengths different from each other is being widely researched to realize field applications or micro systems.
The photomixing method basically uses a beating signal generated by the two lasers different from each other. A beating signal having a THz frequency is generated using two laser signals having wavelengths different from each other to generate an AC current having the THz frequency using the generated beating signal. Here, the antenna integrated in the photomixer generates the THz wave. Accordingly, the generated THz wave has the same frequency as that of the beating signal. Therefore, the THz wave generation apparatus, which is frequency-tunable by adjusting one of the wavelengths of the two lasers, may be manufactured.
In case of a THz continuous wave (CW THz) generator/detector, which is commercially sold at the present time, two independent distributed-feedback (DFB) lasers are used. Outputs of the two DFB lasers, which are operated in wavelengths different from each other, are beaten while overlapping each other in an optical fiber or space. Thus, the CW THz generator/detector uses this phenomenon to generate the THz wave. A frequency variable THz wave generation apparatus in which an operation wavelength of two or one laser is varied to change or to tune the frequency of the THz wave may be manufactured.
Here, one of important physical properties is that stability of the frequency of the generated THz wave depends on wavelength stability of the two lasers used for generating the beating signal. Thus, a noise such as fluctuation of the wavelength of each of the lasers may cause fluctuation of the frequency of THz wave as it is. If the two lasers have wavelengths, which exactly accord with each other in a degree and direction fluctuated by external parameters (e.g., a temperature, an electrical characteristic, or a mechanical vibration), the frequency of the generated THz wave may be stable even though characteristics of the two lasers are not stable.
When two independent lasers are used, there is a limitation that a high expensive wavelength locker should be used in case of a commercial system to minimize instability of the frequency of the THz wave generated through the photomixing method.